1. Field of the Invention
The invention relates to an electric power steering apparatus to assist a steering by a force of an electric motor, more particularly relates to a rack and pinion electric power steering apparatus to assist an axial movable rack shaft by the assisted force.
2. Description of the Related Art
It is typically known a rack and pinion type electric power steering device (EPS) assisting a steering force of a steering wheel, mounted on a vehicle, by a driver. The rack and pinion type EPS acts the assisted steering force to tires through a rack shaft and through a tie rod and a knuckle arm. The rack and pinion type EPS is, for example, disclosed in the Japanese laid-open publication No.11-11333.
The typically known rack and pinion EPS is shown in FIG. 1. An electric power steering device 100 includes a pinion 102 connecting to a steering wheel through an un-illustrated steering shaft. The pinion 102 is rotated by a steering torque caused by a steering of the steering wheel. With the pinion 102 is engaged a rack 106 formed on one end of a rack shaft 104. Both ends of the rack shaft 104 are connected to right and left tires 112 through each of tie rods 108 and knuckle arms 110 and thus the tires 112 are constructed as a steered wheel to be steered by an axial movement of the rack shaft 104.
In the electric power steering device 100, the pinion 102 is rotated by an operation of the steering wheel and a rotating force, that is an assisted torque, of the pinion 102 is transformed into an axial movement force, that is an assisted rotating force or an assisted sting force, of the rack shaft 104. The tires 112 are steered by the axial movement of the rack shaft 104. The pinion 102 is rotatably mounted on a vehicle body to be maintained in engaging with the rack 106 by a rack guide 114. On the other end of the rack shaft 104 is mounted a ball screw mechanism 116 connected to an un-illustrated electric motor. A ball screw nut 120 of the ball screw mechanism 116 is engaged through plural balls with a screw groove 118 formed on a peripheral surface of the other end of the rack shaft 104. The rotating force of the electric motor rotates the ball screw nut 120 to be transformed into the axial movement of the rack shaft 104. The ball screw nut 120 is also rotatably mounted on the vehicle body by a bearing. A torque sensor is equipped between an input shaft and an output shaft of said un-illustrated steering shaft to detect a torque, that is a steering force and a direction of the steering force, rotating the ball screw nut 120 to a direction according to that of the steering.
In said typically known rack and pinion electric power steering device, the driver steers the steering wheel to rotate the pinion 102 in order to move the rack shaft 104 axially. At this moment, the torque sensor detects the steering and the electric motor rotates the ball screw nut 120 in the direction according to that of the steering. Thereby, the assisted steering force into which the torque of the electric motor is transformed acts on the rack shaft 104 to assist the steering force of the steering wheel by the driver.
In the typically known rack and pinion electric power steering device 100, there are a reverse force against the tires from a road and a vibration of an engine and also the steering of tires to cause a free vibration of the rack shaft 104 based on its natural frequency. The rack shaft 104 is supported on the vehicle body by the pinion 102 or the rack guide 114 and the ball screw mechanism 116. Since the pinion 102 and the ball screw mechanism 116 are positioned without relationship to a length of the rack shaft 104, that is without relationship to an amplitude distribution of the free vibration, so that the free vibration causes a deviation in the engagement of the rack 106 with the pinion and also causes a load deviation to the ball screw mechanism 116 to make a knocking noise and to force a life deterioration. The vibration of the rack shaft 104 is transferred to the steering wheel through the pinion 102 to make an uncomfortable steering feeling for the driver.
Said Japanese laid-open publication No.11-11333 discloses a technology to restrain the amplitude by setting a bush in a location of approximately maximum of the amplitude of the vibration in the rack shaft. This disclosure shows only a decrease of the amplitude of rack shaft by increasing a number of supporting points. This disclosure is same to the typically known rack and pinion electric power steering device because a pinion and a ball screw mechanism of this disclosure is located without any relationship to a length of the rack shaft, so that it does not eliminate a deviation in an engagement of the rack with the pinion nor a load deviation to the ball screw mechanism. In this disclosure, the bush restraining the amplitude of the rack shaft is secured to a vehicle body so that the vibration of the rack shaft is transferred to the body by a collision against the bush to decrease a life of an electric power steering device in this disclosure and to cause a knocking noise.
In view of the previously mentioned circumstances, it is an object of the present invention to provide an electric power steering apparatus that restrains a transfer of a vibration of a rack shaft to a vehicle body.
It is second object of the present invention to provide an electric power steering apparatus that eliminates a deviation in an engagement of a rack with a pinion and eliminates a knocking noise.
It is third object of the present invention to provide an electric power steering apparatus that restrains a transfer of a vibration to a steering wheel through a pinion.
It is fourth object of the present invention to provide an electric power steering apparatus that restrains a load deviation against a nut supported by a bearing.
It is another object of the present invention to provide an electric power steering apparatus that further restrains the load deviation by an axial location of a bearing.
During the process of completing the invention, the inventors constructed models for theoretical analysis, digital analysis, geometric analysis, motion analysis, and load distribution analysis of the vibration of the rack shaft to investigate minimum amplitude of the vibration. As a result of this investigation, it was confirmed that each longitudinal location of the pinion and the bearing is positioned in a place corresponding to minimum amplitude of a free vibration of the rack shaft to restrain a transfer of the vibration of the rack shaft to the vehicle body
The invented electric power steering apparatus was completed as a result of this investigation. The electric power steering apparatus according to the invention includes a rack shaft having a rack and a screw groove, a pinion engaging with the rack to move the rack shaft, a nut engaging with the screw groove to be rotated by an electric motor, wherein each longitudinal location of the pinion and the bearing is positioned in a place corresponding to minimum amplitude of a free vibration of the rack shaft. In this invention, when the pinion is rotated by a steering torque based on a steering of a steering wheel connecting to the pinion, the rack shaft having the rack engaged with the pinion is axially moved. Thereby, the steering torque is transformed into the steering force to move axially the rack shaft by the pinion and the rack. At this time, the electric motor rotates the nut according to the steering torque to assist the axial movement of the rack shaft. Thereby, the driving force of the steering wheel by the driver is reduced.
One aspect of this invention is that said each longitudinal location of the pinion and the bearing is positioned in a place corresponding to minimum amplitude of the free vibration of the rack shaft in said neutral position so that the pinion and the nut supported by the bearing are not almost affected by the free vibration. And also, since the amplitude of the rack shaft near the pinion is extremely small, it eliminates a deviation in an engagement of the rack with the pinion and eliminates a knocking noise and it restrains a transfer of the vibration to the steering wheel through the pinion to improve a steering feeling. Since the amplitude of the rack shaft near the bearing is small, it restrains a load deviation acting to the nut supported by the bearing.
The other aspect of this invention is that a location of a rotating center of the pinion is positioned in a state that a distance from an end face of the rack shaft in the neutral position is equal to a length of 0.224L or 0.359L or is within a length from 0.224L to 0.359L wherein the L is a length of the rack shaft, so that the pinion is not almost affected by the free vibration. Because the amplitude of the rack shaft near the pinion is very small, it eliminates the deviation in the engagement of the rack with the pinion and eliminates the knocking noise and also it restrains a transfer of the vibration to the steering wheel through the pinion to improve the steering feeling.
Another aspect of the invention is that a location of an axial center of the bearing is positioned in a state that a distance from the end face of the rack shaft in the neutral position is equal to a length of 0.224L or 0.359L or is within a length from 0.224L to 0.359L, so that the nut supported by the bearing are not almost affected by the free vibration. Because the amplitude of the rack shaft near the bearing is extremely small, it restrains the load deviation acting to the nut supported by the bearing.
More another aspect of the invention is that an axial center of the bearing coincides with an axial center of the nut so that a load acts on the nut supported by the bearing to restrain the load deviation more.